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The sun’s surface spins more slowly than the rest of the star. This may be why

Sunbeams—what a drag. That’s the conclusion of physicists trying to solve a longstanding mystery: why the sun’s surface rotates more slowly than its inner core. The team argues that energy radiating outward from the sun pushes back slightly as it is expelled, providing just enough resistance to put on the brakes. The hypothesis is supported by a new observation: that the thin “skin” of the sun rotates more slowly than layers just beneath.

“I really can’t believe nobody has thought of this,” says Hugh Hudson, a solar physicist at the University of California, Berkeley, who was not involved in the research. “This is a straightforward, simple mechanism nobody noticed before, and it seems to explain a phenomenon no one was able to explain.”

Scientists have known for decades that the sun spins less like a baseball than a soft-boiled egg; it rotates about 5% slower in its outer layer than it does deep inside, creating a shearing motion where the speed changes. But they didn’t know why. The breaking hypothesis is a novel idea that has not been applied to the sun before, says Jeff Kuhn, a physicist at the University of Hawaii’s Institute for Astronomy in Honolulu and lead author on the paper. The premise draws on fundamental ideas like Albert Einstein’s special relativity, which states that photons of light carry momentum, and Isaac Newton’s third law, which stipulates a reaction for every action.

It turns out, Kuhn says, that the reaction from this expelled momentum is enough to slow down the sun. “It slows down from the outside to the inside, and it will gradually slow down all the way to the interior, it’s just a matter of time.” (Not to worry—although the effect is enough to stop the sun eventually, “the slowdown time is longer than the age of the universe,” he adds.)

The idea seems to agree with observations, Kuhn says. Based on the amount of torque being generated by the pushback and the viscosity of the solar plasma, the researchers calculated an expected 2% difference in speed at the shear line between the sun’s 150-kilometer-thick outer skin and the 35,000-kilometer thick layer underneath. Observations from spacecraft such as NASA’s Solar Dynamics Observatory, a space-based telescope that has been making observations of the sun since 2010, showed their prediction to be correct.

In another check, based on the brightness and age of the sun, Kuhn and colleagues calculated how much the outer 5% of the sun should have slowed down over the sun’s entire lifetime. Again, their prediction matched the slowdown that has been observed, they will report next month in Physical Review Letters.

The findings are based on a new way of studying the sun’s tremors and quakes. For years, helioseismologists have used vibrations in the sun to study the sun’s interior. Kuhn’s team developed a new technique for using solar vibrations to study the outer edge of the sun. They used the waves as a “moving marker” to watch the sun’s rotation, just as scientists have watched sunspots progress across the sun’s surface in the past. But whereas sunspots are only visible on the sun’s surface, by watching the waves at the very edge of the solar disc, Kuhn’s team was able to look at them crosswise as they penetrated a few hundred kilometers into the sun’s skin. This gave them a 3D peek into how this outermost layer of the sun was moving over time.

Studying the edge of the sun in this way allowed the team to measure changes on scales as small as 10 kilometers. In contrast, helioseismic data from the interior of the sun could only reveal changes in increments of 2000 kilometers or more, Kuhn notes.

The finding has implications that go beyond our solar system, Kuhn says. “This is a universal effect,” he says. “As long as a star or galaxy is radiating all this energy, it’s going to create this torque, this brake, which slows things down.” That could be a factor in understanding the life cycle of stars, and how stellar rotation affects stars’ magnetic fields. It could be especially relevant to understanding the forces that affect the behavior of the brightest stars, because of their higher levels of energy, he notes.

This is the first time researchers have applied such a granular analysis to the problem and looked at the shear layer close enough to study the slowdown in detail, says Phil Scherrer, a solar physicist at Stanford University in Palo Alto, California, who was not involved with the study. “I think we’ll look back on this and say, ‘Oh wow, that was obvious’—but it isn’t, really.”